This invention relates to air flow control in air handling systems such as heating and cooling systems and, more particularly, to apparatus for providing a constant air flow by a blower used in such systems.
Heating and cooling systems, particularly residential systems are well-known. Such systems commonly employ a blower to push warm or cold air through a duct system depending upon whether the residence is being heated or cooled. The blower is motor operated. The air flow system is designed to operate under a wide variety of conditions which include the volume of space to be heated or cooled, the temperature of the environment, the size and type of ducting used, duct restrictions, etc. One desirable feature of these air handling systems is that they be capable of providing a constant flow rate of air under a wide range of conditions. One method of achieving this constancy is to control operation of the blower motor so its operation varies in response to sensed changes in the system or its operating environment. There are a variety of control schemes which have been employed to control motor operation. Each of these schemes employ a different methodology depending upon the particular desired operating characteristics of the system.
In U.S. Pat. No. 4,978,896, for example, a blower's speed is controlled over a range of static pressure changes to maintain a constant flow rate. A microprocessor is responsive to a preselected flow rate signal and a motor speed signal to generate a motor torque value. A pulse width modulated (PWM) signal is produced by the microprocessor and used to control motor speed so the calculated amount of torque is produced by the motor, enabling the motor to drive the blower at a variable rate which will provide the desired constant air flow rate even though operating conditions change. Co-assigned U.S. Pat. No. 5,019,757 discloses a similar operating scheme but incorporates a comparison function by which the motor's rpm is compared with a calculated target speed with any difference used to adjust an output of the microprocessor to refine the level of control.
Another co-assigned U.S. Pat. No. 4,806,833 describes an air conditioning system in which the blower speed is established to provide a constant volume of air flow for an existing static pressure. The blower speed is subsequently changed in response to changes in the static pressure.
Finally, U.S. Pat. No. 4,476,706, for example, describes an environmental control sytem. The embodiment described is for a tractor cab having two separate blowers, a pressurization blower, and a recirculation blower. The pressurization blower, at constant air flow, is similar to that described in the above co-assigned patents. The occupant of the cab selects the control level of the recirculation blower. The control circuitry is responsive to changes within the cab pressure and system flow restrictions which occur, for example, when an air filter in the system begins to clog up, to maintain a constant flow rate.
Generally, it is a feature of these systems that air flow control, and the maintenance of a constant flow rate of air is a function of motor torque. And, since torque is proportional to motor speed, the slope of the motor's torque/speed curve is proportional to a desired air flow. However, the relationship is not exact, and because of this, the level of control to maintain a desired constant air flow rate, while generally achieved, is not achieved to the greatest extent possible. Also in this regard, control methodologies employing use of the torque/speed curves provide some type of compensation (i.e., gain) in the circuitry used to provide motor drive signals. The gain value used is typically a compromise; and, as a result, is generally excessive for high flow rates of air, and insufficient at lower air flow rates.